Indonesian police in restive Papua on Saturday detained a Ukrainian tourist attending a prayer session to commemorate the 51st anniversary of the region’s movement for independence.

Artem Shapirenko, 36, was detained by police in the town of Manokwari in western Papua where around 50 people took part in a prayer at the traditional leaders council building. It was unclear why he had been held.

Shapirenko, wearing a Bob Marley T-shirt, held his fist in the air and yelled “Free Papua” in Indonesian as police officers ushered him into their vehicle, said an AFP reporter in Manokwari.

A photocopy of the man’s tourist visa, obtained by the police, showed it had expired in July this year.

Papua declared independence from the Dutch on December 1, 1961, but neighboring Indonesia took control of the region with force in 1963. It officially annexed Papua in 1969 with a UN-backed vote, widely seen as a sham.

The separatist Free Papua Movement (OPM), which formed in 1965, also marks the birth of its organization on the December anniversary, when rallies and commemorations are held across Papua.

Police had beefed up security ahead of the anniversary and arrested three youth activists in the city of Jayapura, capital of Papua, according to a provincial police spokesman.

Jakarta keeps a tight grip on Papua and foreign journalists are de facto banned from reporting in the region.

More than 170 people are imprisoned in Indonesia for promoting separatism, most of them from Papua or the Maluku islands in eastern Indonesia, according to Human Rights Watch.